1. Field of the Invention
This invention relates to a method and apparatus for pulverized solid fuel combustion that results in NO.sub.x emissions reduction, and combustion performance improvement, such as improvements in flame stability, turn down capability, etc., in pulverized solid fuel combustion systems. More particularly, this invention is directed to a method and apparatus for pulverized coal (PC) combustion that results in NO.sub.x emissions reduction, and combustion performance improvement, such as improvements in flame stability, turn down capability, etc., in pulverized coal combustion systems. The invention is applicable to pulverized solid fuel combustion processes and systems, and in particular pulverized coal combustion processes and systems, for furnaces, boilers, and other combustion chambers.
2. Description of Prior Art
The utilization of coal and other solid fuels, such as wood waste and biomass, for power generation as well as other applications results in emissions of nitrogen oxides (NO.sub.x) which are formed primarily as a result of oxidation of the nitrogen inherent in the fuel and oxidation of molecular nitrogen present in the combustion air. Nitrogen oxides released in the atmosphere contribute to acid rain, accelerate the photochemical reactions responsible for smog, and result in increased ground level ozone concentrations. NO.sub.x can also be formed when high temperatures (greater than about 2700.degree. F.) are sustained in a flame region where nitrogen and oxygen are present. Under this condition, the molecular nitrogen dissociates and recombines with oxygen forming thermal NO.sub.x.
It is known that lower NO.sub.x emissions can be obtained from pulverized coal flames by staging or delaying the mixing of some of the combustion air with the fuel so that the released nitrogen volatiles combine to form molecular nitrogen instead of NO.sub.x. In the reducing atmosphere produced by staging, molecules of NO.sub.x that do form can also be more readily reduced back to molecular nitrogen. This process of staging may be completed externally to the burner by removing some of the combustion air from the burner and introducing it at another location in the furnace.
The art is replete with processes and devices for combustion of pulverized coal including the combustion of pulverized coal in a manner which reduces NO.sub.x emissions. U.S. Pat. No. 5,908,003 to Hura et al. teaches a process and apparatus for combustion of a solid carbonaceous material in which a mixture of the solid carbonaceous material and combustion air is injected into a combustion chamber and ignited, thereby forming a fuel-lean primary combustion zone, and a gaseous fuel is injected into the combustion chamber downstream of the primary combustion zone, thereby forming a fuel-lean secondary combustion zone. U.S. Pat. No. 5,724,897 to Breen et al. teaches a pulverized coal burner which includes fuel splitters for separation of a mixture of primary air and coal into a plurality of streams while the mixture is discharged through a diffuser having a plurality of partially open areas and a plurality of blocked areas. After passing through the diffuser, the plurality of streams are discharged into a furnace to be burned. U.S. Pat. No. 5,771,823 to Vieistra et al. teaches a method and apparatus for reducing NO.sub.x emissions from multiple-intertube pulverized coal burners in which an internal two-stage process controls the amount of secondary air flow into the burner. The first stage includes the secondary air damper and air flow station to regulate the amount of air which flows into the windbox of the burner. The baffle plate assembly limits the amount of air which flows to the core of the burner for combustion of the fuel by diverting a quantity of air to the periphery of the burner. The second stage includes an outlet formed in the hot primary air ducts, an air plenum which communicates therewith, and a plurality of interjectory air ports which correspond with the burner in number and position along the front wall of the boiler and which communicate with the air plenum. The interjectory air ports inject interjectory air into the combustion chamber of the boiler at a 90.degree. angle to the direction of the burner tips of each burner, supplying the balance of combustion air needed for complete combustion of the fuel. U.S. Pat. No. 5,829,369 to Sivy et al. teaches a pulverized coal burner having a pulverized fuel transport nozzle surrounded by a transition zone which shields a central oxygen-lean fuel devolatilization zone from the swirling secondary combustion air. The transition zone acts as a buffer between the primary and the secondary combustion air streams to improve the control of the air-burner mixing and the flame stability by providing a limited recirculation region between primary and secondary combustion air streams. The limited recirculation regions transport evolved NO.sub.x back towards the oxygen-lean fuel devolatilization zone for reduction to molecular nitrogen. The burner may be configured to fire a combination of fossil fuels, for example, pulverized coal delivered through the primary zone with a small amount of natural gas being injected through the transition zone, wherein the natural gas constitutes between about 5-15% of the burner thermal input. U.S. Pat. No. 5,231,937 to Kobayashi et al. teaches a burner for pulverized coal comprising a coal duct for pulverized coal and primary combustion air and a secondary combustion air duct whereby the coal and primary combustion air and secondary combustion air mix outside the outlet nozzles of the duct and mixing zone where combustion occurs. And, finally, U.S. Pat. No. 5,799,594 to Dernjatin et al. teaches a method and apparatus for combustion of pulverized fuel in a tangentially fired boiler in which an air-deficient mixture of fuel and primary air is introduced through a fuel feeding pipe tangentially into the furnace of the boiler, forming a reducing flame, and at least one stream of combustion air is injected into the furnace.
Although significant strides have been made in the area of pulverized coal combustion to reduce NO.sub.x emissions generated by the combustion of pulverized coal, NO.sub.x emissions from coal fired facilities continues to be problematic. In addition, it is frequently the case that methods and apparatuses for reducing NO.sub.x emissions from heating apparatuses in which the fuel is pulverized coal undesirably impact other elements of the combustion process such as flame stability, turn down capability, CO.sub.2 emissions, and combustion efficiency.